Vacuum infusion is a process wherein vacuum pressure is used to drive resin into a laminate structure. Typically, selected mats of random or woven fabric, such as fiberglass, carbon fiber, KEVLAR®, foam core, or the like, are prepared and enclosed in a vacuum bag. Resin and catalyzer are then infused therein, typically after vacuum is drawn, and polymerization occurs after completion of an optimized curing period and at a selected temperature. The polymerization forms a rigid three-dimensional network structure defined by linear chains with cross-links therebetween.
Positioning of laminate layers is essential to allow for a properly formed structure. Therefore, spray adhesive is sometimes applied to generally hold essentially dry layers in position prior to and during the vacuum infusion process, especially for sloped assemblies, such as large boat hulls. That is, an effective adhesive must be able to hold many layers of reinforcing fabric in a vertical aspect to satisfy the need. Unfortunately, many spray adhesives that are commonly utilized in such manner form a discernible interface, weakening the overall integrity of the cured structure, acting as a contaminant in the matrix. That is, premature failure of the cured structure may result at the area(s) of adhesive application, where resin structure is interrupted.
Resins such as polyester, vinyl ester, or epoxy may be utilized for vacuum infusion. Epoxy resins, however, have better relative mechanical properties and typically produce composite structures that are stronger and more heat tolerant, with a high strength/weight ratio. Epoxy, a structural or engineering adhesive well recognized for excellent adhesion properties and high heat and chemical resistance, finds application as a coating, adhesive and in composite materials, such as those using carbon fiber and fiberglass reinforcements, as discussed further herein. Epoxy is a copolymer comprising resin and hardener. Typically, monomers or short chain polymers with an epoxide group at one end define a resin. Hardener mixes with the resin and its amine groups, such as of the polyamine monomer triethylenetetramine, to form a covalent bond with the epoxide group of the resin. In such manner, a rigid structure is defined with crosslinking therebetween, wherein the modified epoxy adheres to surfaces by forming strong polar bonds therewith.
A majority of epoxy resin is produced from epichlorohydrin and bisphenol-A, wherein bisphenol-A, or phenolacetone, is formed from 2 mole phenol and 1 mole acetone. Epichlorohydrin is a mixture of propylene and chlorine, with free radical substitution at the double bond resulting in allylchloride as a main product, which may be further treated with layer separation and processing. Typically, for liquid epoxy resin, the bisphenol-A,
and epichlorhydrin,
are combined with sodium hydroxide, NaOH, to preferably form epichlorohydrin
releasing Na+ and Cl−. The reaction thus removes unreacted phenol and acetone and attaches two glycidyl groups to the ends of the bisphenol-A to create a standard epoxy resin. The resulting epoxy prepolymer,
is reacted with amine compounds for cross-linking
As noted, spray adhesives typically utilized in the vacuum infusion process to hold laminates together generally influence and negatively influence the successful formation of strong polar bonds between the epoxy and the laminate surface(s). Interruption of the epoxy resin's cross-linking may also occur, further contributing to the weakened interface. That is, as noted, the typical adhesive interface is generally weaker than the rest of the structure, compromising the integrity of the materials formed.
Therefore, it is readily apparent that there is need for a vacuum infusion adhesive that allows for secure placement of laminates and that polymerizes with epoxy resin, thereby creating a seamless cured structure and thereby avoiding the above-discussed disadvantages.